Engineering Safe Shear Motions for the Development of EGS Reservoirs
[Strasbourg Univertsity, France]
The classical application of hydraulic fracturing for the stimulation of oil and gas reservoirs implies the injection of proppant (generally sand) in the hydraulic fracture. The objective is to maintain the hydraulic conductivity of the fracture at a satisfactory value when the in situ fluid pressure drops back to production levels. But the use of proppant in high temperature geothermal reservoirs (larger than 150°C) is limited by chemical interactions between the sand and the geothermal fluids as well as by the thermal stability of the viscous fluids required for injecting the proppant. Shear dilatancy has been proposed as an alternative to hydraulic fracturing. The objective is to build up progressively the pore pressure in the rock mass so as to decrease the effective normal stress supported by critically preexisting fractures. The lower the effective normal stress for which shear motion occurs, the larger the dilatancy associated with shear (Barton et al., 1985), and therefore the larger the permanent increase in hydraulic conductivity of the fracture at the end of stimulation. This concept has been tested in the mid-eighties, in particular at the granite test site of Le Mayet de Montagne ( France) in a 250m to 840 depth range. Results demonstrated the efficiency of the technique but outlined the role of the minimum principal stress magnitude as a limiting factor for the efficiency of the technique. Based on these results, two EGS reservoirs have been developed at the European Geothermal site at Soultz-sous-forêts, the first one at depths ranging from 2800 m to 3600 m, the second one at depths ranging from 4500 m to 5000 m. For both reservoirs it was possible, by increasing progressively the formation pressure, to create fresh shear zones that revealed to constitute key features for the development of these reservoirs. But shear simulations generate micro seismic events, the magnitude of which may not be compatible with the surface use of land. Interestingly, results from Soultz outline the existence of large scale non-seismic shear motions and the presentation will discuss real time monitoring techniques for keeping non seismic the induced shear motions.
|        Topic: EGS - Enhanced Geothermal Systems||Paper Number: 31006|